The instant invention relates to polymers comprising polyester polymer units useful in thermosetting powder coating compositions.
Thermosetting powder coating compositions are well known in the art and are widely used as coatings for office furniture, electric appliances, bicycles, garden furniture, accessories for the automotive industry, general metal parts and the like. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. The powder is applied to the substrate, usually but not limited to a metal, and fused to a continuous film by baking.
Polymers comprising polyester polymer units are useful in thermosetting powder coating compositions. Such polymers comprise a polyester polymer unit containing the formula -(I-III-I)-, or alternatively, -(III-I-III)-, wherein III is derived from a dicarboxylic acid such as terephthalic acid, and wherein I is derived from a diol and/or a polyol such as neopentyl glycol. When 1,4-cyclohexanedimethanol is used as a diol component, the resulting thermosetting powder coating compositions comprising such polymers can suffer from drawbacks related to excessive melt viscosity and processability. Excessive melt viscosity of the polymer of a thermosetting powder coating composition requires an excessive bake oven temperature to fuse the polymer thereby degrading the polymer. As a result, the advantages of using 1,4-cyclohexanedimethanol in a powder coating composition can not be fully realized. The processability of the polyester during its formation is hindered by a successively increasing melt transition temperature as molecular weight builds. This is attributed to the highly crystalline nature of 1,4-cyclohexanedimethanol content. This phenomenon causes difficulties in maintaining a homogeneous polymerization, as plate-out can occur at the edges and interface of the mixture.